JP6558252B2 - High strength ERW steel pipe for oil well - Google Patents

Description

  The present invention relates to a high-strength ERW steel pipe for oil wells. In particular, the present invention relates to a high-strength ERW steel pipe for oil wells having strength equivalent to API standard 5CT P110 and having excellent toughness.

  In recent years, the drilling depth of oil wells and gas wells (hereinafter collectively referred to as oil wells) has tended to become deeper, and in order to increase the crushing strength of casings and the like, higher strength of steel pipes for oil wells is required. .

  Conventionally, seamless steel pipes and ERW steel pipes have been used as steel pipes for oil wells. In the case of a steel pipe that requires high strength, the entire steel pipe is subjected to quenching and tempering after pipe making to ensure strength and improve toughness. On the other hand, recently, with the aim of reducing excavation costs, there has been a strong demand for ERW steel pipes that have high strength and have not been subjected to heat treatment after pipe making, and have been formed.

  Of oil well steel pipes, cheaper ERW steel pipes are used when the strength is relatively low, such as casings near the surface of the earth. In addition, the API standard Spec5CT K55 oil-welded steel pipe for oil wells is manufactured as it is formed by pipe forming (quenching and tempering omitted). Furthermore, the API standard Spec5CT N80-equivalent ERW steel pipe for oil wells may be manufactured as it is. However, when a strength higher than this, for example, a strength equivalent to API standard 5C P110 is required, it is necessary to perform heat treatment after pipe making.

  However, the heat treatment after the pipe making increases the manufacturing cost, and the dimensional accuracy deteriorates due to thermal strain, and a process such as correction may be required again.

  Patent Document 1 discloses an ERW welded steel pipe having a strength and a yield stress equivalent to P110 without increasing the C content for securing strength, without containing B, and without performing heat treatment after pipe forming. The manufacturing method is described. In Patent Document 2, the C content is set within a predetermined range in order to increase strength, yield ratio and toughness, and a uniform structure of bainite is ensured in order to ensure strength, and hardenability is ensured in order to realize these. An electric resistance welded steel pipe as a possible steel component and its manufacturing method are described.

  The electric resistance welded steel pipe described in Patent Document 1 has a structure mainly composed of a low-temperature transformation generation phase represented by ferrite and bainite. For example, an electric resistance welded steel pipe having a thickness of about 7 to 12.7 mm. It is applicable to. However, in shale wells, the drilling depth is further deepened, so that the steel pipe must be made thicker.However, as the wall thickness increases, the cooling rate during hot rolling of the steel material decreases and the transformation temperature increases. There is a risk of insufficient strength.

  In addition, the ERW steel pipe described in Patent Document 2 is cooled after hot rolling to precipitate a bainite structure, and further wound up at 300 ° C. or less to form a hot-rolled steel sheet. Therefore, it is possible to manufacture a steel pipe having a thickness of about 20 mm. However, in the ERW steel pipe described in Patent Document 2, in order to form a bainite structure by continuously cooling the steel sheet at a predetermined cooling rate, an upper bainite generated at a relatively high temperature, and a lower bainite generated at a relatively low temperature, It is presumed that a mixed tissue is obtained. For this reason, the ERW steel pipe described in Patent Document 2 may have a large variation in yield strength.

Japanese Patent No. 5644982 Japanese Patent No. 5131411

  The present invention has been made in view of the above circumstances, and provides a high-strength electric-welded steel pipe for oil wells having strength equivalent to API standard 5C P110, small variation in yield strength, and excellent toughness. This is the issue.

  In Patent Document 2, a uniform bainite structure is obtained by performing accelerated cooling at a cooling rate of 15 ° C./second or higher in a temperature region of 650 ° C. or lower where bainite transformation starts after hot rolling. When the present inventors examined, the structure | tissue obtained by the manufacturing method of patent document 2 was a mixed structure | tissue of an upper bainite and a lower bainite. The cause is that the bainite transformation start temperature of the steel material described in Patent Document 2 is included in the transition boiling region when the steel material is water-cooled, so that the cooling speed varies on the surface of the steel material. It was presumed that mixed bainite was formed due to the precipitation of various types of bainite. For this reason, although it has the intensity | strength equivalent to P110, the yield strength varied with the width | variety of about 150 MPa.

  Therefore, the present inventors have studied that B-containing steel containing B as a chemical component, and performing intermediate air cooling in which the steel sheet is air-cooled at 5 ° C./second or less in the temperature range of the bainite transformation start temperature after hot rolling, Then, when rapidly cooled, a structure mainly composed of upper bainite and including island martensite was obtained. The reason why such a structure was obtained is that the temperature change of the steel sheet during intermediate air cooling is small, so that most of the untransformed structure is isothermally transformed to form a large amount of upper bainite, and then the remaining untransformed structure is formed by rapid cooling. I guess that became island-like martensite. On the other hand, carbon is concentrated in the austenite phase which is an untransformed structure during intermediate air cooling, and coarse island martensite may be generated during subsequent cooling, resulting in a reduction in toughness. Therefore, an attempt was made to lower the finish rolling temperature. When the finish rolling temperature was made relatively low, the effective crystal grains became finer, and it became possible to refine the island-like martensite in a small amount. As described above, the high strength electric resistance welded steel pipe for oil wells of the present invention has a large variation in yield strength because most of the structure becomes upper bainite, and the strength is low because it contains fine and small amount of island martensite. High and excellent toughness.

The gist of the present invention is as follows.
(1) The chemical component is mass%,
C: 0.06 to 0.12%,
Si: 0.40% or less,
Mn: 1.50 to 1.90%,
P: 0.020% or less,
S: 0.0050% or less,
Al: 0.100% or less,
Ti: 0.010 to 0.030%,
Nb: 0.010 to 0.050%,
B: 0.0005-0.000020%
N: not more than 0.010%, with the balance being Fe and impurities,
The structure of the base material part excluding the welded part and the weld heat-affected part is composed of 90 area% or more of upper bainite, 0.5 to 5 area% of island martensite (MA), and the remaining structure.
The major axis of the island martensite (MA) is 2.0 μm or less,
A high-strength ERW steel pipe for oil wells having a yield strength of 760 MPa to 970 MPa, tensile strength of 860 MPa, and yield strength variation of 100 MPa or less.
(2) Furthermore, in mass%,
Ni: 0.50% or less,
Cu: 0.50% or less,
Mo: 0.50% or less,
Cr: 0.50% or less,
V: 0.10% or less,
The high-strength ERW steel pipe for oil wells according to (1), containing one or more of Ca: 0.0050% or less.
(3) The high strength ERW steel pipe for oil wells according to (1) or (2), wherein Charpy absorbed energy at 0 ° C. of the base material part is 60 J or more.

  ADVANTAGE OF THE INVENTION According to this invention, it has the intensity | strength equivalent to API specification 5CT P110, the dispersion | variation in yield strength is small, and also the high-strength ERW steel pipe for oil wells excellent in toughness can be provided.

FIG. 1 is a graph showing the relationship between the finish rolling temperature and the area ratio of island martensite of a steel having a chemical component of the present invention. FIG. 2 is a graph showing the relationship between the area ratio of island-like martensite of steel having the chemical component of the present invention and Charpy absorbed energy. FIG. 3 is a graph showing the relationship between temperature and time in the cooling step after hot rolling. FIG. 4 is a graph showing the relationship between the steel sheet temperature and the bainite transformation rate during secondary cooling. FIG. 5 is a photograph showing the metallographic structure of the steel sheet, where (a) is an example in which intermediate air cooling is not performed, and (b) is an example in which intermediate air cooling is performed. FIG. 6 is a photograph showing the metallographic structure of the steel sheet, in which (a) is an example where the hot rolling completion temperature is 890 ° C., and (b) is an example where the hot rolling completion temperature is 800 ° C.

Hereinafter, the high-strength ERW steel pipe for oil wells of this embodiment will be described.
A high-strength electric resistance welded steel pipe (hereinafter referred to as a steel pipe) according to the present embodiment is a steel pipe formed by electro-welding a steel sheet, and includes a base material portion, a welded portion, and a weld heat affected zone. ing. The chemical composition of the steel pipe is mass%, C: 0.06 to 0.12%, Si: 0.40% or less, Mn: 1.50 to 1.90%, P: 0.020% or less, S: 0.0050% or less, Al: 0.100% or less, Ti: 0.000 to 0.030%, Nb: 0.010 to 0.050%, B: 0.0005 to 0.000020%, N: 0 0.010% or less, with the balance being Fe and impurities. Moreover, the structure of the base material part is composed of 90 area% or more of upper bainite, 0.5 to 5 area% of island martensite (MA), and the remaining structure. Island-like martensite (MA) has a major axis having a size of 2.0 μm or less. The steel pipe of the present embodiment has the composition and structure as described above, so that the strength corresponding to API standard 5C P110 according to the amount of C, that is, the yield strength of the base metal part is 760 MPa to 970 MPa, and the tensile strength is 860 MPa. Have the above. Moreover, the yield strength variation is 100 MPa or less.

  Further, the steel sheet of the present embodiment is further mass%, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, Cr: 0.50% or less, V: 0.5. It is preferable to contain one or more of 10% or less and Ca: 0.0050% or less.

  Furthermore, the steel plate of the present embodiment preferably has a Charpy absorbed energy at 0 ° C. of the base material portion of 60 J or more.

  Hereinafter, the reason which limited the chemical component of the steel pipe of this embodiment is demonstrated. In addition, the description of% means the mass% unless there is particular notice.

(C: 0.06-0.12%)
In the present embodiment, C is an important element for increasing the tensile strength and yield strength of the base material portion and ensuring toughness. In order to make the base material part a structure containing upper bainite and island martensite (MA) and to ensure strength and toughness, the lower limit of the C content is set to 0.06%. On the other hand, when there is too much content of C, the area fraction of the island-like martensite (MA) in a structure | tissue will increase, and toughness will fall. Therefore, the upper limit of the C amount is set to 0.12%. In addition, from the viewpoint of a balance between toughness and strength, the C content is preferably 0.06 to 0.11%, and more preferably 0.07 to 0.12%.

(Si: 0.40% or less)
Si is an element useful for deoxidation and strength improvement. However, if Si is contained in a large amount, the toughness and weldability are deteriorated, so the upper limit is made 0.40%. On the other hand, the lower limit of the Si content is preferably 0.03% in order to ensure a sufficient deoxidation effect. In addition, from the viewpoint of a balance between toughness and strength, the Si content is preferably 0.05 to 0.3%, and more preferably 0.1 to 0.25%.

(Mn: 1.50 to 1.90%)
Mn is an element that improves the strength, and is useful for securing the yield strength and the tensile strength with the structure of the base material portion mainly composed of upper bainite and island martensite (MA). The lower limit of the Mn content is 1.50% in order to sufficiently exhibit the effects of improving the yield strength, tensile strength and low temperature toughness. On the other hand, when Mn is contained in a large amount, the toughness and weldability may deteriorate as in the case of Si, so the upper limit is made 1.90%. More preferably, the Mn content is 1.70% or more.

(P: 0.020% or less)
P is an impurity and is an element that deteriorates low-temperature toughness. Therefore, the smaller the content, the better. However, since it is necessary to balance the cost at the steel making stage with the above characteristics, the upper limit is set to 0.020% in the present embodiment.

(S: 0.0050% or less)
S, like P, is an element that exists as an impurity. The smaller the S content, the better. The reduction of the S content makes it possible to reduce MnS and improve toughness. However, the upper limit is made 0.0050% in consideration of the cost in the steelmaking stage.

(Al: 0.100% or less)
Al is an element usually contained in steel as a deoxidizing material. However, if the content exceeds 0.100%, Al-based non-metallic inclusions increase to impair the steel cleanliness and toughness may deteriorate. Therefore, the upper limit is made 0.100%. In consideration of securing a stable deoxidation effect and the balance of toughness, the Al content is preferably 0.01 to 0.05%.

(Ti: 0.010 to 0.030%)
Ti forms fine TiN and contributes to refinement of the microstructure by suppressing the coarsening of the austenite grains during slab reheating and during the formation of the weld heat affected zone. Moreover, when there is too much N amount mentioned later, since it couple | bonds with B and produces | generates BN, the amount of solid solution B which makes transformation temperature low will reduce. On the other hand, by containing Ti, solid solution N is fixed as TiN and solid solution N is eliminated, generation of BN is suppressed, and solid solution B can be secured. For these purposes, the Ti content is 0.010% or more. On the other hand, if the Ti content is too large, coarse TiN and TiC are generated and the toughness may be deteriorated, so the upper limit is made 0.030%. In addition, Preferably, Ti content shall be 0.010 to 0.020%.

(Nb: 0.010 to 0.050%)
Nb is an element that suppresses recrystallization of austenite and refines the structure during hot rolling. In order to improve yield strength, tensile strength, and toughness, in this embodiment, the lower limit of the Nb content is 0.010%. On the other hand, when there is too much Nb content, there exists a possibility that a coarse precipitate may be produced and toughness may be inhibited, Therefore The upper limit of Nb content shall be 0.050%. In addition, Preferably, Nb content shall be 0.020 to 0.050%.

(B: 0.0005-0.0020%)
In this embodiment, isothermal transformation of bainite is caused by performing intermediate air cooling in the cooling step after hot rolling of the steel sheet, and the metal structure is a structure mainly composed of upper bainite, and the variation in yield strength of the base metal part is varied. Suppress. In order to obtain the upper bainite by causing the isothermal transformation in the intermediate air cooling, it is necessary to lower the bainite transformation temperature of the steel. B is an important element that lowers the bainite transformation temperature in the present embodiment, and the lower limit of the B content is 0.0005%. When the B content is less than 0.0005%, a large amount of ferrite is generated in the metal structure, and a structure mainly composed of upper bainite cannot be formed. On the other hand, if the B content is too large, B-containing precipitates (such as Fe ₂₃ (CB) ₆ ) are likely to be generated, and mechanical properties may vary or toughness may deteriorate. 0.0020%. Preferably, the B content is 0.0006 to 0.0020%, more preferably 0.0009 to 0.0020%.

(N: 0.0100% or less)
N is an impurity, and if the amount of N is too large, TiN increases excessively and may cause adverse effects such as surface defects and toughness deterioration, so the upper limit is made 0.010%. On the other hand, when fine TiN is formed in the steel, the microstructure is refined by suppressing the coarsening of austenite grains in the slab reheating and in the weld heat affected zone, and the low temperature toughness of the base material and the weld heat affected zone Contribute to improvement. In addition, Preferably, N content shall be 0.0020 to 0.0050%.

  In the present invention, in addition to the above elements, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, Cr: 0.50% or less, V: 0.10 % Or less, Ca: One or two or more selected from 0.0050% or less may be included.

Ni is an element that contributes to improvement in yield strength, tensile strength, and toughness. However, Ni is an expensive element, and if the addition amount is too large, the economy is impaired, so the upper limit of the content is preferably 0.50%. A more preferred upper limit is 0.30%. In this embodiment, Ni is a selective element and does not necessarily need to be added. However, in order to stably obtain the above-described effects of Ni addition, the lower limit of the content is 0.04%. Is preferred. Further, since Ni is an element that significantly deteriorates sulfide stress cracking (SSC), it is preferable not to add it when H ₂ S is present.

  Cu is an element effective for improving the strength of the base material portion, but if it is added in a large amount, it may induce surface cracks at the stage of a slab or hot-rolled steel sheet. Therefore, it is preferable that the upper limit of the Cu amount is 0.50%. In the present embodiment, Cu is a selective element and does not necessarily need to be added. However, in order to stably obtain the effect of Cu addition as described above, the lower limit of the content is 0.02%. Is preferred.

  Mo is an element effective for improving the hardenability of steel and obtaining high strength. In addition, Mo coexists with Nb, suppresses recrystallization of austenite during rolling, and contributes to refinement of the austenite structure. However, Mo is an expensive element, and adding too much impairs the economy, so the upper limit is preferably made 0.50%. More preferably, it is 0.20% or less, More preferably, it is 0.10% or less. In the present embodiment, Mo is a selective element and does not necessarily need to be added. However, in order to stably obtain the effect of Mo addition as described above, the lower limit of its content is 0.05%. Is preferred.

  Cr is an element that improves hardenability, and the upper limit of Cr content is preferably 0.5%. In this embodiment, Cr is a selective element and does not necessarily need to be added. However, in order to stably obtain the above-described effects due to the addition of Cr, the lower limit of the content is 0.05%. Is preferred.

  V has almost the same effect as Nb, but its effect is lower than that of Nb. V is an expensive element, and adding too much impairs the economy, so the upper limit of the V amount is preferably 0.10%. In this embodiment, V is a selective element and does not necessarily need to be added. However, the lower limit of the content is more preferably 0.05%, and further preferably 0.03%.

  Ca is an element that controls the form of sulfide inclusions and improves low-temperature toughness. If the Ca content exceeds 0.0050%, CaO-CaS becomes a large cluster or inclusion, which may adversely affect toughness. Therefore, it is preferable that the upper limit of the Ca addition amount be 0.0050%. A more preferable upper limit is 0.0045%. Further, in the present embodiment, Ca is a selective element and does not necessarily need to be added. However, in order to stably obtain the above-described effects due to the addition of Ca, the lower limit of the content is set to 0.0010%. Is preferred. More preferably, 0.0020% or more of Ca is added.

  Further, the remainder other than the above-described elements substantially consists of Fe and impurities. Examples of the impurities include P and S, unavoidable impurities inevitably mixed in the raw material and the manufacturing process, and elements that do not impair the effects of the present invention. These may be contained in a trace amount in the steel pipe of this embodiment.

  Next, the structure of the base material part is composed of 90 area% or more of upper bainite, 0.5 to 5 area% of island martensite (MA), and the remaining structure. The upper bainite is a structure having cementite between lath-like ferrites constituting the bainite structure, and is generated at a relatively higher temperature than the lower bainite. The lower bainite is a structure having iron-based carbide in the lath.

  By making the structure of the base metal part mainly composed of upper bainite, the variation in yield strength of the base material part is significantly larger than that of a mixed structure of upper bainite and lower bainite of less than 90 area%. It is suppressed. As described later, the upper bainite is formed by isothermal transformation of untransformed austenite during intermediate air cooling after primary cooling. When the area fraction of the upper bainite is less than 90%, the area fraction of the remaining structure such as the lower bainite and ferrite is relatively increased, and the yield strength and tensile strength of the base metal portion are decreased, and the yield strength is not uniform. Since it occurs, it is not preferable.

  Island-like martensite (MA) is formed by transformation of untransformed austenite during rapid cooling after intermediate air cooling. Since the toughness is lowered when the island-shaped martensite is coarsened, in this embodiment, it is preferable to limit the major axis of the island-shaped martensite to 2.0 μm or less, and more preferably to 1.0 μm or less. If the major axis exceeds 2.0 μm, the toughness is lowered, which is not preferable. Further, if the area ratio of the island martensite is less than 0.5 area%, the yield strength decreases to less than 760 MPa, which is not preferable. Moreover, since the toughness will fall when the area ratio of island-like martensite exceeds 5 area%, it is unpreferable. Island-like martensite is hardly formed in a conventional mixed structure of upper bainite and lower bainite, and is easily formed in a structure mainly composed of upper bainite as in the present embodiment. When it is difficult to distinguish between upper bainite and lower bainite, it can be presumed that the structure is mainly composed of upper bainite by confirming the presence of island martensite.

  The remaining structure is not particularly limited, and examples thereof include retained austenite, lower bainite, and ferrite. Even if the entire remaining structure of the steel pipe is the lower bainite, it is included in the present invention.

  The main phase upper bainite, island martensite, and the remaining structure constituting the metal structure of the base metal part are identified and the area fraction is measured using the Nital reagent and the reagent disclosed in JP-A-59-219473. Then, the position of 1/4 of the plate thickness t (t / 4 position) in the cross section in the tube axis direction or the cross section perpendicular to the tube axis direction of the base material portion corresponding to the position 90 ° from the seam portion is corroded. Can be observed by scanning and transmission electron microscopes of 1000 to 100,000 times. Moreover, you may measure the same cross section by EBSD method (electron beam backscattering electron diffraction method).

  For example, the upper bainite and the lower bainite have different carbide generation sites and crystal orientations (elongation directions), so the inside of the lath-like crystal grains is measured using a field emission scanning electron microscope (FE-SEM). It can be easily determined by observing the elongation direction of the iron-based carbide. Moreover, the island-like martensite can observe the structure | tissue of t / 4 position with a 1000 times image of EBSD (IQ) by EBSD method, and can make the place observed black into island-like martensite. The position of the island-like martensite can be determined by checking the position of the island-like martensite by repelling and etching the same section in advance and attaching it to the EBSD (IQ) image.

The area fraction of the upper bainite, island-like martensite, and the remaining structure is obtained by taking a sample with the plate thickness section parallel to the rolling direction of the steel plate as the observation surface, polishing the observation surface, corroding with the Nital reagent, The area fraction is measured by observing the range of 1/8 to 3/8 thickness centering on 1/4 with FE-SEM. The area ratio is measured at 10 fields at a magnification of 5000 times, and the average value is defined as the area ratio.
Further, the major axis of island martensite is obtained by measuring the major axis in a 1000-fold image of EBSD (IQ) for 10 fields of view and averaging this.

  Further, the yield strength and tensile strength of the base material part are measured by collecting a test piece from the position of the base material part corresponding to the position of 90 ° from the seam part. For the test piece, an arc-shaped tensile test piece having a full thickness in the longitudinal direction of the steel pipe is taken in accordance with API5CT.

  Moreover, the yield strength variation is divided into three equal parts along the longitudinal direction, and the 90 ° position from the seam part of the pipe end in the traveling direction of the steel pipe taken from each end (front and rear part) of the coil divided into three equal parts. A total of twelve arc-shaped tensile specimens were collected from a position of 270 °, the yield strength was measured for each specimen, and the difference between the maximum value and the minimum value of the obtained yield strength was regarded as the variation in yield strength. To do.

  Moreover, Charpy absorbed energy prepares a test piece based on API 5CT and measures Charpy absorbed energy at a temperature of 0 ° C. The obtained value is converted to full size based on API 5CT.

Next, the manufacturing method of the steel pipe of this embodiment is demonstrated.
The steel pipe of this embodiment hot-rolls a steel slab having the above chemical components, completes finish rolling at an Ar3 point or higher and 830 ° C or lower, and is primary up to 500 to 600 ° C at a cooling rate of 10 to 70 ° C / s. Cool, perform secondary air cooling at a cooling rate of 5 ° C./s or less for 5 seconds or more, perform cooling at a cooling rate of 10 ° C./s or more, take up at 200 ° C. or less to obtain a hot-rolled steel sheet, This hot-rolled steel sheet is manufactured by electroforming and welding the abutting surfaces while forming into a tubular shape.

  The finish rolling temperature is preferably from Ar3 point to 830 ° C. If the finish rolling temperature is less than the Ar3 point, ferrite starts to form in the metal structure, which is not preferable. Further, when the finish rolling temperature exceeds 830 ° C., carbon is concentrated in untransformed austenite during secondary cooling, and excessively hard island-like martensite is generated in the subsequent tertiary cooling, which greatly reduces toughness. . In FIG. 1, the relationship between the finish rolling temperature of the steel which has the chemical component of this invention, and the area ratio of an island-like martensite is shown. FIG. 2 shows the relationship between the area ratio of island martensite and Charpy absorbed energy. As shown in FIG. 1, it can be seen that when the finish rolling temperature is 830 ° C. or lower, the area ratio of island martensite is 5 area% or lower. Moreover, as shown in FIG. 2, when the area ratio of an island-like martensite becomes 5 area% or less, it turns out that Charpy absorbed energy becomes 60J or more. By setting the finish rolling temperature to 830 ° C. or less, the effective crystal grains are refined, thereby reducing the size and area ratio of island martensite which is an embrittled phase, and toughness is improved.

  In addition, Ar3 point can be calculated | required from the thermal expansion behavior at the time of heating and cooling using the test material of the same component as a hot-rolled steel plate. Moreover, it is also possible to obtain | require by the following (Formula 1) from the component of a hot-rolled steel plate.

  Ar3 (° C.) = 910-310C-80Mn-55Ni-20Cu-15Cr-80Mo (Formula 1)

  Here, C, Mn, Ni, Cu, Cr, and Mo are content [mass%] of each element. Ni, Cu, Cr, and Mo are arbitrary additive elements in the present invention. When these elements are not added intentionally, the calculation is made as 0 in the above (Equation 1).

  After hot rolling, in order to obtain a structure having the upper bainite as the main phase, it is rapidly cooled to the bainite transformation start temperature. Specifically, as primary cooling, water cooling is performed to 500 to 600 ° C. at a cooling rate of 10 to 70 ° C./s. If the cooling rate is less than 10 ° C./s, the area fraction of ferrite may increase, such being undesirable. Moreover, even if it cools at a cooling rate over 70 degreeC / s, since an effect is saturated, an upper limit shall be 70 degreeC / s. Moreover, when the cooling stop temperature of the primary cooling is less than 500 ° C., the secondary cooling temperature is lowered, and there is a possibility that the lower bainite may be generated excessively. Moreover, when the cooling stop temperature of primary cooling exceeds 600 degreeC, since secondary cooling temperature becomes high and there exists a possibility that the area fraction of a ferrite may increase, it is unpreferable.

  After the primary cooling, intermediate air cooling is performed as secondary cooling for 5 seconds or more at a cooling rate of 5 ° C./s or less. The start temperature of the intermediate air cooling is almost the same as the stop temperature of the primary cooling. By this intermediate air cooling, the untransformed structure (austenite structure) after the primary cooling is transformed to bainite. The transformation rate at this time is 80% or more. When the cooling rate exceeds 5 ° C./s, the steel plate temperature is greatly lowered during intermediate air cooling, and lower bainite is generated, which is not preferable. Moreover, if the intermediate air cooling time is less than 5 seconds, the transformation rate is less than 80%, and the area fraction of the upper bainite is decreased, while the area ratio of the island martensite and the remaining structure is increased. The upper limit of the intermediate air cooling time is preferably 10 seconds or less. When the intermediate air cooling time exceeds 10 seconds, the formation of island martensite is hindered and the material is substantially tempered, which is not preferable because the yield strength and tensile strength of the base material portion are lowered.

  FIG. 3 shows the relationship between the cooling time and the steel plate temperature. Conventionally, when the cooling after finish rolling is continuously performed at a cooling rate of 10 to 70 ° C./s, it passes through a transition boiling region during cooling, but at the same time passes through a temperature region where bainite transformation occurs. For this reason, in the conventional manufacturing method, variation in the cooling rate occurs in the steel plate passing through the transition boiling region, and the steel plate temperature continues to decrease while passing through the temperature region where bainite transformation occurs. A mixed structure is easily obtained. On the other hand, in the present embodiment, by performing intermediate air cooling at a higher temperature than the transition boiling region and at a low cooling rate, the decrease in the steel plate temperature when passing through the temperature region where bainite transformation occurs is reduced, and the upper bainite is reduced. Is preferentially generated, and variations in intensity are reduced. Moreover, since the transformation rate in intermediate air cooling becomes 80% or more, the area fraction occupied by the structure other than the upper bainite becomes small, and the toughness is hardly lowered.

  Moreover, in FIG. 4, the relationship between the steel plate temperature at the time of secondary cooling and a bainite transformation rate is shown. Reference numeral 1 is a curve showing the behavior of the transformation rate when steel (comparative steel) containing no B (boron) is rapidly cooled from over 600 ° C. Moreover, the code | symbol 2 is a curve which shows the behavior of a transformation rate at the time of rapidly cooling steel (comparative steel) containing B (boron) from more than 500 degreeC. In both cases of reference numerals 1 and 2, the transformation rate increases as the cooling proceeds from a high temperature to a low temperature, but the slope of the rising curve is relatively gentle. For this reason, in the code | symbol 1 and 2, the mixed structure | tissue containing a ferrite and an upper bainite or an upper bainite and a lower bainite will be formed. On the other hand, the code | symbol 3 is a curve which shows the behavior of the transformation rate at the time of carrying out the intermediate air cooling of the steel (invention steel) containing B (boron) at the cooling rate of more than 500 degreeC to 5 degrees C / s or less. In reference numeral 3, it can be seen that the rising curve is steeper than in reference numerals 1 and 2, and the transformation rate rapidly increases from 0% to nearly 100% within a narrow temperature range. Thus, in the manufacturing method of the steel pipe of this embodiment, by performing intermediate air cooling at a cooling rate of 5 ° C./s or less, the transformation rate rapidly increases in a narrow temperature range so that a structure mainly composed of upper bainite can be obtained. become.

  After the secondary cooling, tertiary cooling is performed at a cooling rate of 10 ° C./s or more, and winding is performed at 200 ° C. or less to obtain a hot rolled steel sheet. By the tertiary cooling, the remaining untransformed structure becomes fine island martensite, and the strength of the base material portion is improved. When the cooling rate of the tertiary cooling is less than 10 ° C./s, it is difficult to form island martensite and the strength of the base material portion is lowered, which is not preferable. On the other hand, when the coiling temperature exceeds 200 ° C., the island-shaped martensite is decomposed and the strength is lowered, which is not preferable.

  Next, in the present embodiment, the obtained hot-rolled steel sheet is air-cooled, formed into a tubular shape in the cold, and the end portions are butted together and electro-welded to manufacture the steel pipe of the present embodiment. In the present embodiment, the thickness and the outer shape of the steel pipe are not particularly specified, but the ratio t / D between the thickness t of the steel sheet and the outer diameter D of the ERW steel pipe is about 2 to 8%, and t is It can be suitably applied to those having a size of 5 mm to 16 mm.

  Furthermore, you may perform the seam heat processing which heats only an electric-welding welding part and accelerates cooling. In ERW welding, the butt portion is heated and melted, and pressure is applied to join the vicinity. Therefore, the vicinity of the ERW weld portion is plastically deformed at a high temperature and then rapidly cooled. Therefore, the ERW welded portion is harder than the steel plate, and the low temperature toughness and deformation performance of the ERW steel pipe can be further improved by performing seam heat treatment.

  FIG. 5 (a) shows a metallographic photograph of a steel sheet having the chemical composition of the present invention, which was rapidly cooled at a constant cooling rate from hot rolling to winding and not subjected to intermediate air cooling. . FIG. 5 (b) shows a metallographic photograph of a steel sheet having the chemical composition of the present invention, which is rapidly cooled from hot rolling to winding and subjected to intermediate air cooling. As shown in FIG. 5 (b), it can be seen that by performing intermediate air cooling, a structure containing upper bainite as the main phase and including island martensite is obtained. On the other hand, as shown to Fig.5 (a), when intermediate air cooling is not implemented, it turns out that it becomes a mixed structure of an upper bainite and a lower bainite, and an island-like martensite is not formed.

  FIG. 6 (a) shows a metallographic photograph of a steel sheet having the chemical composition of the present invention and having a hot rolling completion temperature of 890 ° C. FIG. 6 (b) shows a metallographic photograph of a steel sheet having the chemical composition of the present invention and having a hot rolling completion temperature of 800 ° C. As shown in FIG. 6B, by reducing the hot rolling temperature, the area fraction of island martensite becomes 1.8%, and the major axis becomes 1.5 μm. On the other hand, as shown in FIG. 6A, when the hot rolling temperature is high, the area fraction of island martensite is 12.5%, the major axis is 2.2 μm, and the toughness is reduced. I understand that.

  Hereinafter, the effect of the present invention will be described in detail with reference to examples. The present invention is not limited to the conditions used in the following examples. Moreover, the blank in Table 1 indicates that the element is not intentionally added. Steels A to G are steels that satisfy the definition of the component composition of the present invention, and steels AA to DD are steels that do not satisfy the definition of the component composition of the present invention.

  Steels A to G and AA to DD having chemical components shown in Table 1 were cast into steel pieces. These steel pieces were heated to the heating temperature shown in Table 2, hot-rolled at the rolling finishing temperature shown in Table 2, and cooled to obtain a hot-rolled steel sheet. In addition, all the rolling finishing temperatures shown in Table 2 are temperatures of Ar3 point or higher. The cooling process includes primary cooling for cooling to 500 to 600 ° C. at a cooling rate of 20 to 30 ° C./second, and secondary cooling for cooling at an intermediate air cooling temperature, an intermediate air cooling time and a cooling rate of 5 ° C./s or less shown in Table 2. Cooling and tertiary cooling which cools at a cooling rate of 20 to 30 ° C./s were performed. Then, it wound up at the winding temperature shown in Table 2, and was set as the hot rolled sheet steel.

  Next, after the obtained hot-rolled steel sheet was air-cooled, it was formed into a tubular shape in a continuous roll forming process, and the ends of the hot-rolled steel sheet were butted together and subjected to electric resistance welding. Thereafter, if necessary, a seam heat treatment for accelerated cooling was performed after heating the ERW weld.

  Next, from the obtained ERW welded steel pipe, a sample for observing the structure is taken, and a cross section parallel to the longitudinal direction of the steel pipe is subjected to nital etching, and observed with a scanning electron microscope and a transmission electron microscope. It was measured by the method (electron beam backscattered electron diffraction method). Specifically, a sample is taken with the cross section of the plate thickness parallel to the rolling direction of the steel plate as the observation surface, the observation surface is polished, corroded with the Nital reagent, and 1/8 to about 1/4 of the plate thickness. By observing the 3/8 thickness range with FE-SEM, the area fraction of upper bainite, island martensite, and the remaining structure was measured. The area ratio was measured at 10 fields at a magnification of 5000 times, and the average value was defined as the area ratio. Moreover, the island-like martensite observed the structure | tissue of t / 4 position with the 1000 times image of EBSD (IQ) by EBSD method, and identified the location observed black as island-like martensite.

  Further, the yield strength and tensile strength of the base material part were measured by collecting test pieces from the base material part corresponding to the position 90 ° from the seam part. As the test piece, an arc-shaped tensile test piece was taken in the longitudinal direction of the steel pipe in accordance with API5CT.

  Further, the yield strength variation is divided into three equal parts along the longitudinal direction, and the 90 ° position and 270 ° position from the seam portion of the pipe end in the traveling direction of the steel pipe taken from each end portion of the three divided coils. A total of twelve arc-shaped tensile test pieces having a full thickness are collected, the yield strength is measured for each test piece, and the difference between the maximum value and the minimum value of the obtained yield strength is defined as the variation in yield strength.

  Moreover, Charpy absorption energy prepared the test piece based on API5CT, measured Charpy absorption energy in the temperature of 0 degreeC, and obtained value converted into full size based on API5CT. The L direction in Table 2 is a case where a test piece along the tube axis direction is prepared. Moreover, C direction is a case where the test piece along the circumferential direction of a steel pipe is prepared. When the plate thickness was 10 mm or more, the test piece was collected along the C direction (the direction perpendicular to the tube length), and when the plate thickness was less than 10 mm, the test piece was collected along the L direction (the tube length direction).

  As shown in Table 2, the steel pipes of test numbers 1 to 7 have a structure in which the metal structure includes upper area bainite having 90% by area and island form martensite having 1 to 5% by area, and yield strength (YS). In addition, the tensile strength (YS) is high, and the variation in yield strength (YS) is 100 MPa or less. Moreover, the Charpy absorbed energy (CVN) at 0 ° C. is 50 to 80 J. Thus, the steel pipes of test numbers 1 to 7 have yield strength and tensile strength equivalent to API standard 5C P110, have small variations in yield strength, and are excellent in toughness.

On the other hand, since the steel pipe of Test Example 8 was not subjected to intermediate air cooling, the structure of the base metal part became a mixed structure including upper bainite and lower bainite, and the variation in yield strength was increased.
In the steel pipe of Test Example 9, the finish rolling temperature exceeded 830 ° C., and as a result of the concentration of carbon in the austenite, the area ratio of island martensite was 13%, and the Charpy absorbed energy was significantly reduced.
In the steel pipe of Test Example 10, the intermediate air cooling temperature (primary cooling stop temperature) was too high at 650 ° C., and therefore, 30 area% of ferrite was formed in the structure of the base material portion, and the yield strength and tensile strength were reduced. In addition, Charpy absorbed energy also decreased significantly.
In the steel pipe of Test Example 11, since the intermediate air cooling temperature (primary cooling stop temperature) was too low at 400 ° C., 70 area% lower bainite was formed together with the upper bainite in the structure of the base metal part, and the variation in yield strength was Increased.
In the steel pipe of Test Example 12, the intermediate air cooling time was as short as 3 seconds, the bainite transformation did not proceed sufficiently during the intermediate air cooling, and lower bainite was generated during the tertiary cooling. Thereby, 60 area% lower bainite was formed together with the upper bainite in the structure of the base material portion, and the variation in yield strength was increased.

Since the steel pipe of Test Example 13 had a low B content, the bainite transformation temperature of the steel increased, ferrite was generated during primary cooling after hot rolling, and yield strength and tensile strength decreased.
Since the steel pipe of Test Example 14 had a low C content, yield strength and tensile strength were reduced.
In the steel pipe of Test Example 15, since the C content was too high, the area ratio of island martensite was 8%, and the Charpy absorbed energy was greatly reduced.
The steel pipe of Test Example 16 had low yield strength and tensile strength because the Mn content was low.

Claims (3)

Chemical composition is mass%,
C: 0.06 to 0.12%,
Si: 0.40% or less,
Mn: 1.50 to 1.90%,
P: 0.020% or less,
S: 0.0050% or less,
Al: 0.100% or less,
Ti: 0.010 to 0.030%,
Nb: 0.010 to 0.050%,
B: 0.0005-0.000020%
N: not more than 0.010%, with the balance being Fe and impurities,
The structure of the base material part excluding the welded part and the weld heat-affected part is composed of 90 area% or more of upper bainite, 0.5 to 5 area% of island martensite (MA), and the remaining structure.
The major axis of the island martensite (MA) is 2.0 μm or less,
A high-strength ERW steel pipe for oil wells having a yield strength of 760 MPa to 970 MPa, tensile strength of 860 MPa, and yield strength variation of 100 MPa or less. In addition,
Ni: 0.50% or less,
Cu: 0.50% or less,
Mo: 0.50% or less,
Cr: 0.50% or less,
V: 0.10% or less,
The high-strength ERW steel pipe for oil wells according to claim 1, containing one or more of Ca: 0.0050% or less.   The high-strength ERW steel pipe for oil wells according to claim 1 or 2, wherein the base metal part has Charpy absorbed energy at 0 ° C of 60 J or more.